Microorganisms play an important role in the mining industry where they are now used in the bioleaching recovery of copper, uranium, and gold (see Rawlings, D. E., S. Silver [1995] "Mining With Microbes," Biotechnology 13(August):773-778). Until now, microorganisms have not been used in conjunction with commercial chemical leach processes.
Copper sulfide (Cu.sub.2 S) is an acid soluble metal sulfide that is a component in the mineral known as chalcocite. Currently in the mining industry, a solution of ferric sulfate (Fe.sub.2 (SO.sub.4).sub.3) is used to solubilize the copper from chalcocite by producing soluble copper sulfate (CuSO.sub.4), along with insoluble elemental sulfur (S) and soluble ferrous sulfate (FeSO.sub.4). The reactions involved, in summary form, are: EQU Cu.sub.2 S+Fe.sub.2 (SO.sub.4).sub.3.fwdarw.CuS+CuSO.sub.4 +2FeSO.sub.4 EQU CuS+Fe.sub.2 (SO.sub.4).sub.3.fwdarw.CuSO.sub.4 +S+2FeSO.sub.4
The stripped leach solution, or raffinate, from this process contains ferrous sulfate (FeSO.sub.4). Ferrous sulfate is a product having little value or utility and is often a waste product from the leaching process. A means to rapidly convert the ferrous sulfate waste back to ferric sulfate would enhance the efficiency of the chemical leaching process as well as providing an environmentally beneficial effect.
Johnson et al. ([1988] Mining Engineering December:1119-1122) describe a microbial process by which the conversion of ferrous sulfate to ferric sulfate can be accomplished. In the biological process, ferrous sulfate (FeSO.sub.4) is acidified with sulfuric acid (H.sub.2 SO.sub.4). Then, in the presence of oxygen (O.sub.2) and carbon dioxide (CO.sub.2), iron-oxidizing bacteria restore the iron sulfate to its oxidized ferric sulfate form. The reactions, in summary form, are: ##EQU1##